Originally Posted by Photo Engineer
Imagine a paper safe made from red plastic. Paper is not sensitive to red light. Therefore, the safe is 100% light tight as far as paper is concerned but it will fog film.
Of course there are a lot of alternative scenarios for your observation.
Another issue, some plastics degrade over time, so an older plastic paper safe, may still be safe enough for B&W paper, but not for colour paper or film.
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The greatest advance in photography in the last 100 years is not digital, it's odourless stop bath....